A typical conventional mobile tank truck used for cleaning a grease trap, such as a restaurant underground grease trap for the entrapment of cooking oil, grease and the like, of a water disposal system is shown in FIG. 1. Such a system comprises a closed elongated, cylindrical tank vessel 10 mounted horizontally on a wheeled base bed 11 of a truck 12. The tank 10 defines a single interior chamber or compartment which is accessible for cleaning through top and rear opening access hatches 14, 16. An intake port 17, connected to a diagonally rising intake pipe 18 (see FIG. 2), serves as a connection for a hose 19 to draw an aqueous slurry of grease and water from the grease trap into the interior of tank 10. A deflector 20 is located at the exit of pipe 18. A discharge port 21, located in the cover of the rear hatch 16, serves to drain the same slurry from the tank 10 after removal to a remote disposal site. A rotary vane vacuum pump 23, connected through a moisture trap or scrubber 24 and a shutoff assembly 25 to the interior of tank 10, serves to reduce or increase pressure in the tank 10 for control of drawing or expelling the slurry into or out of the tank 10. A pressure relief valve 27 acts to guard against the hazardous buildup of unacceptable pressure within the tank 10, and one or more sight eyes 28 enable the visual monitoring of the tank filling or discharging process.
In the existing grease trap cleaning procedures which utilize a mobile tank truck of the type illustrated in FIG. 1, the entire aqueous slurry is removed from the trap and transported to an approved disposal site. This is wasteful for several reasons. The entire volume of slurry in the grease trap is removed and takes up the same volume in the tank, regardless of the grease to water ratio in the trap. This means that the tank will be filled by the same amount, and the same amount must be transported and handled for disposal, regardless of whether the trap is filled with 90% grease and 10% water, or 10% grease and 90% water. For a 4,000-gallon tank vessel 10 emptying 1,200- to 3,000-gallon grease traps, therefore, the tank is brought to capacity and a trip to the disposal site is required after as few as one to three grease trap emptying stops. It is, thus, considered desirable to be able to separate the grease from the water at time of loading, so that the number of grease traps that can be cleaned before reaching full tank capacity is a function of the volume of grease, not the total volume of grease and water, in the loaded slurries.